1. Field of the Invention
The present invention relates to key switch devices adapted to be used in keyboards for operating computers, cash registers, electronic typewriters, and other electronic apparatuses.
2. Description of the Related Art
A key switch used in a keyboard is constructed such that when its key top is depressed by a user's finger, a conductor member therein touches fixed electrodes, thereby turning the switch on. Once the user's finger is removed from the key top, the key top is restored its original position by the resilience of a return spring.
Conventional key switches generally make use of metallic return springs. However, in U.S. Pat. No. 4,659,879, a key switch has been disclosed which uses a rubber spring, while Japanese Patent Disclosure No. 54-29209 discloses a key switch wherein a spongy elastic member is interposed between a rubber spring and a conductor member.
The key switch disclosed in U.S. Pat. No. 4,659,879 comprises a housing, an inverted-cup-shaped rubber spring located in the housing, a conductor member within the rubber spring, fixed electrodes placed opposing the underside of the conductor member, a plunger located above the rubber spring, and a key top mounted on the upper end of the plunger.
In the key switch containing the rubber spring, the rubber spring bends when the key top is depressed by a user's finger, so that the conductor member touches the fixed electrodes, thereby turning the switch on.
After the switch is turned on, the force of depression continues to act on the key top, by virtue of the force exerted by the user's finger. By this time, however, the fixed electrodes are already being touched by the conductor member, with the result that the rubber spring ceases moving.
The distance from the point corresponding to the instant the conductor member touches the fixed electrodes to the end of the stroke is what is called the over-stroke. For reason of user operability, the overstroke of a key should preferably be 0.9 mm or more. Conventional key switches, however, often have an overstroke as short as about 0.5 mm. As a result, the key touch is often so dull that the operator cannot always be sure that completion of the switching operation has been achieved, and must therefore consciously determine whether or not the switching operation has been completed, throughout the time he or she is operating the keyboard. Such a conscious effort inevitably results in the operator quickly becoming tired and diminishing his/her work efficiency. This problem requires a solution which cannot be provided by the use of the rubber spring disclosed in Japanese Patent Disclosure No. 54-29209.